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Microwave Connector Considerations for Defense UAVs

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by Kelvin Riddle, Business Development Manager, RF & Microwave – Nogales, Winchester Interconnect

Military and aerospace missions rely on unmanned aerial vehicles (UAVs) for intelligence, security, and reconnaissance (ISR), electronic warfare, and communications systems, and they must function with the highest reliability possible, no matter how severe the operating conditions. High quality interconnections, including those operating at RF and microwave frequencies, allow UAVs to provide optimum results by linking cameras, sensors, and other data collecting subsystems to control centers. The connectors carry power, data, voice, and video and must handle extreme shock, vibration, and temperature with little or no disruption or degradation in performance.

Defense UAVs are often designed and assembled with commercial-off-the-shelf (COTS) components such as coaxial connectors as such components are lower in cost than military grade components, although built and tested for equivalent reliability. COTS interconnections are compatible with MIL-SPEC or MIL-STD connectors and 100% mating compatibility with military grade connectors.

They are compatible when they meet or exceed the latest military reliability specifications. They are environmentally sealed and have electromagnetic interference (EMI) shielding to protect against crowded signal environments. Multiple-mission UAVs may require many connect/disconnect cycles to accommodate different missions. Connectors for those UAVs should be designed to handle many connect and disconnect cycles (100 or more) with minimal or no degradation in performance or reliability.

Figure 1: MQ-9 Reaper drone

These early UAVs for defense and aerospace applications feature modular architectures with flexibility for different mission requirements. The MQ-9A Reaper, for example (Figure 1), can carry multiple mission payloads, including electro-optical/infrared (EO/IR) systems, multiple radar systems, electronic support measures (ESM) systems, laser scanning systems, and weapons packages. It also uses GPS and GLONASS signals for positioning purposes. Some of the latest versions of the UAV enable in-field modifications thanks to a fully modular payload with reliable coaxial interconnections.

Choosing UAV Connectors

RF and microwave coaxial connectors for military UAVs require the same electrical, environmental, and mechanical characteristics as high-frequency military grade connectors, and their payloads differ little from the electrical and mechanical requirements of high-frequency connectors onboard aviation equipment. They must withstand elevated levels of shock and vibration across wide operating temperature ranges. For an ISR UAV, connector size and weight are often starting points in the specification process since minimal size and weight help designers achieve the reduced size, weight, and power (SWaP) required to extend a UAV’s operating range and flight time. Various connector configurations are used for operating frequencies from RF through millimeter-wave frequencies at various power handling and intermodulation-distortion (IMD) limits, including Type-N, BNC, TNC, SMA, and 2.92-mm connectors.

The applications will determine the required frequency range served, with a frequency range of DC to 40 GHz serving a wide range of communications, EW, and radar applications. Low VSWR, such as 1.5:1 or better, is important in any mounting configuration over the entire frequency range, including enclosures and semirigid and flexible cables. Low connector loss is also essential, especially for UAVs in which signal loss translates into a reduced operating range. As UAVs and their electronic systems often operate at millimeter-wave frequencies, connectors with smaller dimensions for those smaller wavelengths, such as 2.92 mm, are typically incorporated for frequency coverage to 40 GHz.

Electrical interconnections within defense UAVs are not generally considered for other applications and should be checked. UAVs may operate in adverse weather conditions, with dust in the air and water splashing on equipment surfaces. Connectors constructed with appropriate environmental sealing can be selected according to the level of protection the sealing provides.

Figure 2: The MVP™ series of connectors are available with SMA, TNC, Type N, and 2.92-mm interfaces and constructed with the features needed for use in defense drones

A defense UAV may require that microwave connectors be hermetically sealed to protect the electrical contacts against chemicals, moisture, sand, and salt spray. Various defense documents provide details on the types of construction methods and ruggedization materials that are proven to provide high reliability where hermeticity is recommended or advised, including MIL-STD-348, MIL-PRF-39012, and MIL-PRF-55339. For applications requiring limited or no use of hazardous materials, these documents also provide guidance on which materials to avoid in specific applications.

Connectors can be compared according to their ingress protection (IP) code, which indicates the amount of dust and water protection achieved by an environmental seal. Typical IP codes for coaxial connectors include IP64 for protection from dust and some splashed water, IP67 for protection against dust and temporary immersion under water, IP68 for protection against dust and prolonged immersion in water, and IP69K for protection against dust and high-pressure water and steam.

Operating temperature range is an essential consideration for connectors in UAVs as they may operate in a wide range of environments, from frigid cold to the scorching heat of desert outposts. More damaging than the high or low temperatures is any kind of rapid temperature change in which circuit materials such as PCBs and packaging will be subjected to contraction with cold and expansion with heat. Connectors are tested to maintain consistent VSWR and low loss across wide frequency ranges while mounted in temperature test chambers. They are available with wide temperature ranges, typically -25° C to +125° C but as wide as -55° C to +200° C and even wider.

Connectors can be constructed with integral EMI and radio-frequency-interference (RFI) structures where signal leakage and interference are concerned. In some defense applications, interconnections may also be subjected to high electromagnetic pulse (EMP) energy, distorting performance and even damaging the connectors.

Figure 3: The PowerSnap power connectors are available in SMT/thruhole and cable-mount configurations capable of handling as much as 40 A current

To ensure dependable contacts while subjected to elevated levels of shock and vibration, coaxial connectors employ several types of locking approaches, such as push-pull mechanisms, screw threads, and bayonet locking mechanisms. Each approach is designed to prevent the two halves of a connector assembly from decoupling during these conditions. While such locking mechanisms can prevent decoupling under harsh operating conditions, they can also have limited lifetimes for enduring locking and unlocking procedures. They should be considered when selecting connectors that may require large numbers of connector mating and demating cycles.

Surveying Solutions

As an example of a coaxial connector with the required qualities for many UAV internal microwave interconnections, the MVP™ (moisture and vibration proof) series of connectors from Winchester Interconnect features SMA, TNC, Type-N, or 2.92-mm interfaces (Figure 2). The connectors incorporate gold-plated beryllium/brass contacts and stainless steel connector bodies for DC to 40 GHz frequency coverage.

The company also provides threaded connectors and can customize interconnect solutions to 110 GHz for UAVs requiring tight fits, such as connectors with edge-launch, low profile, and ganged configurations. The threaded connectors feature a unique Spiralock® thread geometry that provides equal loading of each thread, minimizing loss from the loading of the first few threads. The MVP connectors employ a 1 to 5 oz.-in. torque friction mechanism that is simple and effective, eliminating the need for lock wires.

The MVP connectors are moisture ingress rated to IP68 by employing a clamp nut and O-ring seal that forms a watertight seal to the cable jacket. Strain relief of the cable is provided by a simple slide-on boot that snaps securely to the connector. MVP assemblies also have solder clamps and three-piece attachment design configurations for high-frequency performance.

They are designed and tested at operating temperatures from -40° C to +125°C and are rated for a minimum of 500 mating cycles. When attached to a suitable low-loss coaxial cable, they have a VSWR of 1.35:1 or less from DC to 40 GHz. With an IEC 60527 liquid and solid-particle ingress rating, the connectors are evaluated for endurance of mechanical vibration according to measurements made per EIA-364-28, condition V, test G.

Connectors provide the means of routing energy to different subsystems for DC power, and they must be secure. Power connectors are typically available in two- and three-contact wire-to-wire and wire-to-board configurations, with current carrying capacity as one of the performance parameters determining the type of power connectors used in a particular UAV.

Winchester Interconnect offers power solutions with its PowerSnap connectors. They employ a snap mating structure for SMT/thru-hole and cable mount configurations and can handle currents up to 10 A. The mated PowerSnap connector halves (Figure 3) form a low profile component that ensures constant and reliable connections under harsh operating conditions.

Stable, dependable high-frequency and power interconnections can keep a UAV flying and ensure its electronic systems operate as intended. As they become more significant members of worldwide armed forces units, they also assume increasing importance in many other markets, including commercial, industrial, and medical applications. The reliability of the interconnections, whether for RF and microwave signals or power, must be certain as lives may depend upon it, even when a UAV may be flying in less than ideal conditions.

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